Picking back up on his blog, Sean Coates shares a Dashboard widget he's created to do quick and dirty PHP interpreting on the fly:

I found myself using php -r on the command line to test some simple code snippets, and it occurred to me that this isn't "The Mac Way." So, following the excellent documentation at Apple, I managed to come up with something pretty useful in an afternoon.

His widget sits happily, ready to take in PHP code and output the result at the click of a button.